The present invention relates in general to electronic devices and, more particularly, to semiconductor devices used in battery protection circuits.
Batteries are used in a wide variety of applications including digital pagers, cellular telephones, and in general, portable computer electronics. Batteries employed in portable electronics include lithium ion, lithium polymer, or nickel-cadmium compositions. Many of the portable electronic devices have a battery protection circuit which senses a battery cell voltage, the charge and discharge of current, and maintains the battery cell device within operating specifications. When the battery is charged up to a maximum potential, a control switch changes state and a voltage polarity switch occurs. Thus, the battery protection device can be exposed to a negative voltage resulting in electrical damage if the device is not able to tolerate a negative voltage. Because of this negative voltage, it is desirable to have a battery protection circuit which can handle a negative voltage applied to source terminals of N-type Metal Oxide Semiconductor Field Effect Transistor (NMOSFET) devices which may be contained within the battery protection circuit.
In the prior art, an NMOSFET device whose source can handle a negative voltage has an N-type substrate with a P-type well region formed within the N-type substrate. The N-type substrates, however, are not common on conventional Complementary Metal Oxide Semiconductor Field Effect Transistor (CMOSFET) technologies. The conventional technology uses a P-type substrate for MOSFET devices. A second example of a device whose source can handle a negative voltage has a P-type substrate, an N-type epi region formed within the substrate, and a P-type well region formed within the N-type epi region. The second example has a device which uses a P-type substrate used in the conventional CMOS technology. The second example, however, has a disadvantage in that the process is expensive and does not have good isolation between devices built using the same substrate. Typically, in such epi technologies, the devices are separated from each other by a deep P-type region formed within the N-type epi region. The added region adds complexity and an unnecessary expense. Further, an N-type epi region must also be formed in the P-type substrate. The P-type well region being formed within the N-type epi region.
Thus, a need exists to have an MOSFET device for battery protection circuits that can handle a negative voltage on its source, with respect to the substrate potential of the MOSFET device, using a P-type substrate without the added expense of the prior art devices. The invention disclosed herein will address the above problems.